What is the Difference between a Software License Agreement and a 'Software as a Service' (SaaS) Agreement?

The act of people buying and downloading software (or purchasing it on CD) is very general nowadays. And with these activities, a software license agreement is being concluded. But, as new technologies emerge, new business models appear, and they need a different legal background and a different kind of legal documents to protect them. The “Software as a Service” (SaaS) model is becoming one of the most general business models, as more and more companies use it, from small companies to big corporations.1 But what is SaaS, and why is it different from a more typical software license agreement from a legal perspective? And what are the main issues that must be regulated in an SaaS agreement?